1. Field of the Invention
This invention relates in general to certain new and useful improvements in thermal control surfaces for use on substrates subjected to outer space environment, and more particularly, to substrates having a thermal control surface which enables a high absorption of solar radiation along with a high emission of heat in order to obtain an absorption to emission ratio of approximate unity.
2. Brief Description of the Related Art
In space environments, there is no atmosphere used for conducting heat away from a spacecraft or for that matter in conducting heat to a spacecraft. Therefore, any heat gain or heat loss in an object in a outer space environment must be achieved by radiation. Moreover, the use of solar radiation for purposes of heating, and even more importantly for removing heat from a spacecraft is accomplished through the use of thermal control coatings on a surface of an object. In this way, the thermal control surface can absorb certain solar radiation or otherwise emit or radiate outwardly certain radiation to the space environment.
Thermal control coatings can be formulated and applied in such a manner so as to a have desirable value for solar absorptance (.alpha.) and a desirable value for infrared radiation emittance (.epsilon.). In many cases, it is important to maintain a touch temperature within an allowable design range such that touching of the surface by an astronaut will not cause bodily injury. This is particularly true in the case of micrometeoroid shields. Thus, these micrometeoroid shields require a coating which absorbs as much solar radiation as possible while radiating as much heat away from the substrate as possible to the space environment such that an .alpha./.epsilon. ratio of unity is achieved.
The solar absorptance properties and the infrared emittance properties of any thermal control coating must also be stable in order to maintain the temperature of the spacecraft in a range required for effective operation. However, for spacecraft which are in an orbit near the earth, (commonly referred to as the "low-earth orbit" or "LEO") these spacecraft experience a very hostile space environment. In the low-earth orbit, the spacecraft or other object is consistently bombarded by atomic oxygen, ultra-violet radiation, charged particles, and for that matter, they are degraded by contamination from other spacecraft components. It has been well established that these factors will degrade the optical properties of a spacecraft thermal control surface fairly quickly in a low earth orbit space environment.
Because of the above problems, there has been a need for a suitable long-life thermal control coating for longevity of any spacecraft structure. Moreover, the coating must be at least moderately economical to produce and easy and economic to apply to or form on a spacecraft structure and it must be easy to handle. Heretofore, there have been attempts to use various types of organic and inorganic black paints to provide the desired optical properties. However, these black paints typically are attacked in the low-earth orbit by those mentioned above and hence, the optical properties very quickly degrade and the paints erode in this environment.
Anodic coatings of aluminum are a very attractive type for a thermal control coating because of the light weight of the anodic coating. Moreover, an anodic coating is integral with the aluminum substrate. Furthermore, the anodic coating does not spall or chip, even when impacted by micrometeoroid debris. In addition, an anodic coating is completely resistant to erosion from atomic oxygen. However, one of the serious deficiencies with the acid formed anodic coatings on an aluminum substrate is the very substantial increase in the solar absorptance by the coated substrate without the concomitant infra-red radiation emission, thus raising the temperature of the control surface to an unacceptable level in the low-earth orbit space environments.
A thermal control coating of the type having a high absorptance so that it is capable of absorbing heat, such as solar heat, in a dark side of an orbit and a high emissivity for radiating the heat on the exposed side of an orbit would be highly desirable. There is presently no known coating which is capable of actually providing an absorptance to emission ratio of 1 and which is still stable in a space environment, although that is the desired goal. Thus, there is a need for a component which would maintain a relatively constant temperature controlled to have at least sufficient heat for operation and comfort to an astronaut but not excessive heat which would cause burning of an astronaut in a low earth orbit environment.